Anodizing of aluminum and aluminum alloys is accomplished by immersing articles to be anodized in an electrolyte, with the articles being connected to the positive terminal of a power source and with the cathode electrode, which can be one or more electrode elements or the tank itself, being connected to the negative terminal of the power source. Anodizing may be accomplished in a batch (static) system in which articles are statically disposed in the electrolyte, or in a continuous (dynamic) system in which articles are moved into successive tanks of electrolyte, with continual replenishing and progression of articles in and out of the tanks.
The properties of the oxide film obtained by anodizing of aluminum and aluminum alloys depend on the type of electrolyte employed. Anodizing in water solutions of weak acids, such as boric acid, citric acid, etc., and often referred to as barrier layer anodizing, provides thin (up to one micron) homogeneous oxide films with good dielectric properties. The term weak acid refers to an acid which does not appreciably dissolve the oxide film. Anodizing in water solutions of strong acids, that is, electrolytes which are capable of appreciably dissolving the oxide film, such as sulfuric acid, chromic acid, oxalic acid, etc. provides a thick (from several up to hundreds of microns) porous oxide film with decorative, corrosion and chemical resistant and other intended properties.
A major distinction between anodizing in strong electrolytes and anodizing in weak electrolytes is the necessity, when using strong electrolytes, of cooling the electrolyte to a low operating temperature typically in the range of 0.degree. to 24.degree. C. The cooling apparatus usually includes cooling coils immersed in the electrolyte and electrically grounds the electrolyte. Barrier layer anodizing is usually accomplished at a relatively high temperature, typically near 100.degree. C., and does not ordinarily require any cooling.
For the provision of relatively thick, hard, wear-and-corrosion-resistant aluminum oxide film on the surfaces of aluminum and alloys thereof a process of hard anodizing or hard coating is employed in which a strong electrolyte is maintained at a relatively low operating temperature and relatively high voltages employed to achieve the intended oxide coating. Such hard anodizing techniques are described, for example, in U.S. Pat. Nos. 2,743,221; 2,855,350; 2,987,125; 2,905,600 and 2,977,294 of the present assignee. For hard anodizing, the potential can be up to 70 V or higher which presents a shock hazard to personnel when handling articles without turning off the voltage. Such handling often occurs when articles are simultaneously run in a tank and removed after different anodizing times to achieve different coating thicknesses and also when loading or unloading endless conductive conveyor belts. At higher voltages, the anodizing process becomes more vigorous, leading to burning of articles entering the electrolyte at such higher voltages. For hard coating, a voltage of up to 70 VDC or more must be applied to the articles at the end of the process and this voltage will inevitably lead to burning if the process is started with this high voltage. Step anodizing in a succession of tanks with a step rise in voltage in each tank is not possible, if, as in conventional systems, the electrolyte in the tanks is grounded and thus has the same zero potential and if the conveyor carrying articles through the tank is conductive and therefore is at another single potential.